This invention relates to all voltage controlled oscillators (VCOS) and particularly those used in phaselocked loop systems.
A voltage controlled oscillator is a device whose output frequency (Fout) is a function of its input voltage (Vin). A VCO is often used to track a reference signal. It does this by using a phase-locked loop. A phase detector compares the phase of the reference input signal (Fref) and the divided VCO output signal (Fout/N, N=1, 2, 3, 4, . . . ). Depending on this phase difference, the input node of the VCO is either charged or discharged, thereby altering the VCO's frequency of oscillation. When the frequencies of the two phase detector input signals are matched within a given error margin, the PLL is then said to be "locked" to the reference signal. VCO design is critical in determining the overall performance of a PLL. The VCO's stability to small fluctuations in its input voltage is very important in determining PLL stability and jitter. Jitter can be thought of as spurious variations in the time positions of a series of successive pulses. Let's now look at the typical characteristic of a VCO.
The VCO characteristic shown in FIG. 1 assumes for simplicity sake that as the DC input voltage (Vin) increase, the output frequency (Fout) also increase. Of course, the reverse characteristic also exists. While temperature and power-supply variations will cause the oscillation frequency to change (indicated by &lt;-&gt; in FIG. 1), the far biggest influence on where the characteristic ultimately lies and its slope is the actual process condition ("bst", "typ" and "wst") at the time of manufacture. "bst" represents the process condition (resistor value, transistors parameter values, wire capacitance value) which results in highest output speed, "typ" represents those values which result in average output speed while "wst" represents those values which result in lowest speed. FIG. 1 assumes typical temperature and power-supply conditions.
The input voltage ranges (Va, Vb, Vc) which produce the actual required output target Fout(target) frequency range depend heavily on the process condition. Consequently, the current range (Ia, Ib and Ic) shown in the I-Fout characteristic of FIG. 2) which a current controlled oscillator requires to produce oscillations in the Fout(target) frequency range changes drastically from one process condition to another. The required current range (Ic) is widest in the worst ("wst") process condition and narrowest (Ia) in the best ("bst") process condition.
FIG. 3 shows a conventional VCO. The conventional VCO 140' comprises a bias generator 170' for generating a bias voltage in response to a DC input voltage (Vin) and a single current controlled oscillator 180' for generating an output signal having an output frequency (Fout) in response to the bias voltage. Such conventional VCOs are disclosed in, for example, U.S. Pat. No. 4,876,519 issued to Craig M. Davis et al., U.S. Pat. No. 5,331,295 issued to Jules J. Jelinek et al., U.S. Pat. No. 5,359,301 issued to Anthony B. Candage, U.S. Pat. No. 5,300,898 issued to Dao-Longe Chen et al., and U.S. Pat. No. 5,298,870 issued to Christopher Cytera et al.
Traditionally, irrespective of the process condition, a fixed current range (Itrad) has been used to produce the required Fout(target). See FIG. 2. Hence, the same current range is supplied to the VCO 140' in FIG. 3 in the worst case where a large current range (Ic) is required to produce oscillations in the Fout(target) frequency range as it is in the best case where in contrast only a small current range (Ia) is needed. It will now be illustrated with an example that using this traditional approach, small fluctuations in input current can produce significant shifts in output frequency and hence, increase jitter and degrade system stability.
For example let's assume that:
(a) Itrand=250 .mu.A, Ia=20 .mu.A, Ib=40 .mu.A and Ic=90 .mu.A, PA0 (b) the VCO operates over a Ftarget=120 MHz frequency range, PA0 (c) that a 50 mV fluctuation in input voltage produces a (Itrand/50) variation in input current.
This 5 .mu.A change in input current will produce a 30 MHz, 15 MHz and 6.67 MHz change in oscillation frequency for a "bst", "typ" and "wst" case process condition, respectively. In fact, the steeper the slope (gain) of the I-Fout characteristic, the bigger the frequency shift. Consequently, system stability is degraded and jitter increase.